Layer waviness and debonding are common manufacturing imperfections in thick carbon fiber composite components. Layer waviness may significantly reduce the compression strength of the composite component. For example, waviness in a critical portion of a wind turbine blade, such as the spar cap, may lead to failure of the blade, resulting in the loss of the equipment and power generation capability.
The blade of a wind turbine is primarily loaded in bending due to aerodynamic lift forces. To resist bending, a pair of spar caps are placed as far apart as possible in the flapwise direction on the back sides of the upwind face and the downwind face. The spar caps conventionally include unidirectional fibers running along the length of the blade. To be effective, the spar caps are connected to each other by a shear web made of diagonal fibers.
The problem of waviness may have been a critical issue in reported wind turbine manufacturing instances where the layers of the laminates in carbon composite spar caps were found to be affected by in-plane and/or out-of-plane waviness and disbonds. In order to reliably predict the structural integrity and strength of wind turbine blades with the potential for internal waviness and disbonds, the flaws must be detectable and characterized using a non-destructive testing (NDT) method.
X-ray Computed Tomography (CT) scanning is the most reliable NDT method in terms of detecting fiber waviness. CT images provide detailed information about the fiber distribution that is, so far, not achievable with other NDT techniques. The main advantage of CT imaging is its ability to locate and size planar volumetric details of features in three dimensions. The volumetric information significantly enhances the characterization of features with complex geometries, such as waviness. CT, however, is very expensive, requires significant safety precautions, is not applicable for massive quality control, and is not practical for on-site inspection.
Ultrasonic tests (UTs), on the other hand, have been successfully implemented for field inspections to detect and size debonding, porosity, and voids in composite laminates, but it is unreliable for features with complex shapes. Its detection and characterization abilities for waviness are limited. Traditional A-scans, B-scans, and C-scans only provide two-dimensional (2-D) information of the waviness. Since multiple 2-D B-scan and C-scan images must be integrated mentally to give a sense of any out-of-plane waviness, 2-D viewing of three-dimensional (3-D) complex features, such as waviness, using conventional 2-D B-scans and C-scans, may significantly hamper the ability of such an NDT system to quantify and visualize structures with a complex geometry.
Due to the complex geometry of internal organs and anomalies in the human body, 3-D medical ultrasonic imaging has gained significant attention in the past few years and has played an increasingly important role in diagnosis, minimally invasive image-guided interventions, and intra-operative imaging. Researchers and commercial companies in the medical imaging field, including GE Healthcare (Little Chalfont, UK), are increasingly integrating 3-D visualization into the ultrasonic instrumentation. This advanced imaging tool, however, has not been fully implemented in commercial UT software for routine NDT inspection of critical components.